Apply `login_not_required` decorator to all webhook views

[Zerotask]

When you use Django's LoginRequiredMiddleware, all URLs are private by default and you have to add a login_not_required_middleware to make an URL public. Therefore webhook views like BrevoBaseWebhookView should use the decorator
@method_decorator(login_not_required, name="dispatch")

Django and some other 3rd party libraries already adding this decorator to specific views like LoginView or PasswordResetView

[medmunds]

Seems like this would need to apply to all webhooks, not just Brevo. I think this is the right thing to do (based on this Django forum topic), but the release notes around LoginRequiredMiddleware are frankly a little sparse on guidance for third party libraries. [EDIT: asked in the Django forum to be sure.]

Since we're still supporting Django 4.2, we'll also need either a backported login_not_required or a dummy implementation.

In the meantime, it should be possible to apply login_not_required in your own project, either by subclassing the Anymail webhook view to add the decorator or (possibly) directly in your urls.py with something like login_not_required(BrevoTrackingWebhookView.as_view()).